The coils and capacitors used in filters with frequencies above a few hundred MHz can be reproduced by line sections having suitable length and designated impedances. While the inductances of the designated value can be realized relatively easily by short line sections with high impedances, in production, it is virtually impossible to accurately maintain capacitance values of a few pF using inner conductors coupled in a capacitive manner. The tolerances of the produced capacitance values adversely influence the frequency response. This phenomenon is illustrated graphically in FIG. 2, showing a plot of reflection vs. frequency for the circuit diagrammed FIG. 1. Specifically, Curve 1 shows the frequency-dependent curve of the reflection factor for the setpoint values of the inductances and the capacitances. Curve 2 shows the progress of the reflection factor at a deviation of the capacitance values by 10% from the setpoint value. As a result of this difficulty, multiple-circuit filters are usually produced in stripline technique and with adjustable trimming capacitors. Respective junctions are necessary at the input and output for insertion into coaxial antenna feeder cables.
Conventional single-stage filters include a first inner conductor coaxially disposed with respect to a second inner conductor. The opposite surfaces of the first and second inner conductors are dimensioned such that a series capacitance having a set value is formed (optionally in conjunction with a dielectric other than air). If the capacitance value needs to be adjustable, one of the inner conductors must be adapted to telescope so that it can be displaced with respect to the other inner conductor. Typically, the displaceable part of this inner conductor contacts its fixed part in a conductive manner, e.g., using spring segments. Such a configuration produces intermodulation products at the contact points. These products are undesirable, especially in technologies such as cellular radio, which requires a high signal-to-intermodulation ratio.